Collision is an important issue in a typical multiple access communication system. It has been observed experimentally that the throughput will be severely reduced to its half or even further when several stations are accessing a common channel frequently without detecting and preventing from collisions in advance. In other words, a collision-detection operation prior to accessing the common channel and a channel re-access mechanism after collisions occur will improve throughput of a multiple access communication system, for example HomePNA 2.0, significantly.
For HomePNA 2.0, a constant TRN16 sequence is designed to fulfill the purpose of early detection. The receiver end of a HomePNA 2.0 system uses a copy of TRN16 sequence to correlate or match with the received signal and identify unexpected peaks at its output to declare collisions. It typically involves complex numerical operations to implement such correlation means.
As described in “Interface Specification for HomePNA 2.02.7 10M8 Technology”, all packets shall consist of a header section in which the PREAMBLE64 is defined as repetition of four 16-symbol sequences so-called as TRN16. FIG. 1 shows the frame structure of HomePNA 2.0 for transmitting/receiving signals. The TRN16 sequence results from encoding 0xfc483084 into a sequence of 4 QAM signal constellations, as shown in FIG. 2, in the order of MSB bit transmitted first at 2 MBaud, 2 bits-per-Baud, with the scrambler disabled. The TRN16 is a pseudo-random, constant amplitude QPSK sequence. The preamble was designed to facilitate the following: (1) power estimation and gain control, (2) baud frequency offset estimation, (3) equalizer training, (4) carrier sense, and (5) collision detection.
Please refer to FIG. 3 which is a schematic block diagram showing a conventional collision detection apparatus. The apparatus interfaced between a station 10 and a system channel 20 includes an adaptive equalizer 11, a signal processing device 12, a match filter 13, a collision detection circuit 14 and a channel accessing device 15. When the apparatus receives a signal, the received signal is transmitted to both of the adaptive equalizer 11 and the match filter 13. The signal equalized by the adaptive equalizer 11 is further transmitted to both of the signal processing device 12 and the match filter 13. In the signal processing device 12, the signal is detected and processed by slicing at epoch n to obtain an error term e(n) which is used to update the equalizer's coefficient adaptively. Meanwhile, the equalized preamble is compared with the originally received one in the match filter 13. The comparing result is outputted to the collision detection circuit 14 to determine whether collisions occur. The channel accessing device 15 then controls whether and when the station 10 can use the system channel 20.
If there is only one station transmitting signal onto the channel, four distinct sharp spikes can be observed by a receiver at its match filter output, as illustrated in FIG. 4A. On the other hand, if an unexpected HomePNA 2.0 type signal in addition to the transmitted signal appears at along front end, the receiver shall observe additional peaks at other symbol instants, as shown in FIG. 4B.
A HomePNA 2.0 compliant system including a transmitter and an associated receiver can use above features to identify collisions at its receiver output when it is transmitting a signal.
Basically, the correlator 13 operates as follows:       O    ⁢          (      k      )        =            ∑              n        =        0            15        ⁢                  r        ⁢                  (                      k            -            n                    )                    *              c        ⁢                  (          n          )                    in which                c(k);k=0, . . . , 15 denotes the TRN16 sequence.r(t)=s(k)+i(k)+n(k)=received signal at either equalizer's input or its output sampled at epochs k.wheres(k)=sr(k)+jsi(k) represents expected signal,i(k)=ir(k)+jii(k) denotes interference, andn(k) is a channel noise.        
The timing resolution of a collision can be doubled if the received signal is sampled at twice the symbol rate, and c(k) is interpolated with zero at every other symbol. In this case, the output now becomes       O    ⁢          (      k      )        =            ∑              n        =        0            31        ⁢                            r          2                ⁡                  (                      k            -            n                    )                    *                        c          2                ⁢                  (          n          )                    in which                c2(k); k=0, . . . , 31 denotes the TRN16 sequence interpolated with zero at every other symbol.        r2(k)=received signal at either equalizer's input or its output sampled at twice the symbol rate at epochs k.        
The above-mentioned conventional collision detection method, however, requires an additional match filter of complicated circuitry to achieve the purpose of correlation. Accordingly, the cost will be relatively high.